Process for producing a magnesium die-casting alloy



Nov. 13, 1951 R. F. HAUSER 2,575,273

PROCESS FOR PRODUCING A MAGNESIUM DIE CASTING ALLOY Original Filed Jan. 31, 1945 ATTORNEY /1 Patented Nov. 13, 195i PROCESS FOR PRODUCING A MAGNESIUM DIE-CASTING ALLOY Richard F. Hauser, Bergenfield, Bendix Aviation Corporation,

corporation of Delaware Application December 5, 1947, Serial No. 789,908

The present invention relates to a novel process for producing a die casting of a magnesium alloy, and this application is a continuation-in-part application combining the subject matter of copending applications Serial No. 575,568, filed J anuary magnesium alloy having superior corrosion re-v sistance due in part to its manganese content and also to its low impurity content caused by the settling out in large part of such impurities through the novel method employed in preparing the alloy.

Another object of the invention is to provide a novel processfor preparing a magnesium alloy for use in die casting.

Another object of the invention is to provide a novel method of preparing a magnesium alloy in which there is added to a magnesium-manganese alloy an amount of aluminum to decrease the solubility of the manganese in the alloy so as to precipitate a portion of the manganese out of solution tending to cleanse the melt of suspended iron particles while the temperature of the melt is maintained sufiiciently high so as to prevent reduction of the manganese below a predetermined desired value.

Another object of the invention is to provide a novel die cast magnesium alloy having a substantially homogeneous equi-aXed grain structure.

Another object of the invention is to provide a novel process for treating a die cast magnesium alloy.

Another object of the invention is to provide a novel heat treatment or artificial aging process for increasing the yield strength and hardess of die castings of this alloy.

Another object of the invention is to provide a process for causing an equi-axed grain structure to occur with resultant improved ductility and stability to the material. By the term equiaxed is meant the approximate equal dimensions of the grain with relation to their own axes and in relation to each other.

Other objects and advantages of this invention are set forth in the following description and the novel features thereof arepointed out in the ap- 5' Claims. (01. 148-3) N. J., assignor to Teterboro, N. J a

2 pended claims. The disclosure, however, is illustrative only and changes may be made therein within the principle of the invention to the full extent indicated by the broad and general meanings of the terms in which the appended claims are expressed.

In Figure 1 there is illustrated a photo-micrograph of a typical grain structure obtained in this alloy in the as cast condition and before aging.

In Figure 2 there is illustrated a photo-micrograph of a typical grain structure obtained in this alloy after the heat treatment or artificial aging process explained hereinafter and forming the subject matter of the present application.

Example 1 It has been found, as disclosed in my copending application Serial No. 575,568, filed January 31, 1945, now abandoned, that for certain types of magnesium die castings, the alloy may be prepared by the following process:

A binar magnesium-manganese alloy containing approximately 1.5% manganese by weight is first melted, and to the molten metal there may be added 1.25% to 2% aluminum by weight at approximately 1250-1350 degrees Fahrenheit.

The addition of this amount of aluminum will decrease the solubility of the manganese in the magnesium and the manganese may be reduced to between .6% and 1.0% by weight. The melt is held at this temperature for an appreciable period so that the manganese which is precipitated out of solution tends to cleanse the melt of suspended iron and other particles by causing them to sink to the bottom of the melt due to agglomeration of these particles to the precipitated primary manganese.

This action is not to be confused with the heretofore known settling process in which the precipitation of manganese depends primarily upon holding the melt for long periods of time at relatively low temperatures. In the instant process the temperature of the alloy should never be allowed to drop below 1250' degrees Fahrenheit before being cast or frozen, as such a decrease in temperature below 1250 degrees Fahrenheit during the settling process would cause an adverse reduction of the manganese. It is well to retain from about .6% to 1.0% manganese in order to still further increase the corrosion resistance of the alloy.

The magnesium alloy thus obtained will have an aluminum content by weight of between 1.25% to 2%, a manganese content by weight of from 13% to 1.0% and impurities or other ingredients by weight not in excess of a .3% maximum.

Upon completion of the settling process, the thus-cleansed magnesium-manganese-aluminum alloy is removed and may be directly die cast or may be cast into pigs for convenience in handling. In the latter event, the pigs, as desired, are reheated to a temperature of between 1250 degrees and 1350 degrees Fahrenheit for pouring into a die casting mechanism of conventional type.

Due to the short time for diffusion during solidification, sufiicient deviation from equilibrium conditions are attained to permit a small amount of the hardening material or eutectic to be retained at the grain boundaries of the solidified metal. Precipitation of the eutectic, an aluminum magnesium compound, is later accentuated by artificial aging at temperatures of approximately 360 F. However, the amount of this material in the grain boundaries is not sufficient to cause excessive hot-shortness or embrittlement of the die casting at elevated temperatures, since the aluminum content is preferably maintained at a value not in excess of 2% by weight. It has been found that with an aluminum content in excess of 2% hot-shortness or embrittlement at elevated temperatures becomes so pronounced that it would be very difficult to make such die castings without hot-cracking occurring as the casting is ejected from the die.

It is important, however, that the aluminum content of the alloy for these types of castings be maintained at a minimum of about 1.25% to assure good physical properties in the subsequent die castings and assure against internal porosity being increased to such an extent as to cause the castings to be unsound.

The typical physical properties of this alloy in the die cast and artificial aged condition are:

Yield strength 1bs./sq. in 18,000 Tensile strength lbs./sq. in..- 30,000 Elongation per cent 8 Yield strength is defined as the stress at which the stress-strain curve deviates .2% from the modulus line.

Example 2 For certain other types of magnesium die castings, of more complex design, it has been found, as disclosed in my copending application Serial No. 666,759, filed May 2, 1946, that the alloy may be prepared as follows:

As in the previous example, a binary magnesium-manganese alloy containing approximately 1.5% manganese by weight, may be first melted and to the molten metal there may be added 1.00% to 1.5% aluminum by weight at approximately 1250-1350 degrees Fahrenheit.

The addition of this amount of aluminum will decrease the solubility of the manganese in the magnesium and the manganese may be reduced to between .5% and 1.0% by weight. The melt isheld at this temperature for an appreciable period depending upon size of melt, for example, a minimum of fifteen minutes for a thousand pound melt, so that the manganese which is precipitated out of solution tends to cleanse the melt of suspended iron and other particles by causing them to sink to the bottom of the melt due to agglomeration of these particles to the precipitated primary manganese, as previously described;

In the instant process the temperature of the alloy should never be allowed to drop below 1250 degrees Fahrenheit before being cast or frozen, 76.

as such a decrease in temperature below 1250 de-- grees Fahrenheit during the settling process might cause an adverse reduction of the manganese. It is important to retain between .5% and 1% by weight of manganese in order to still further increase the corrosion resistance of the alloy.

The magnesium alloy thus obtained will have an aluminum content by weight of between 1.0% to 1.5%, a manganese content by weight of from .5% to 1.0%. and impurities or other ingredients by weight not in excess of a 3% maximum.

Upon completion of the settling process, the thus-cleansed magnesium-manganese-aluminum alloy is removed and may be directly die cast or may be cast into pigs for convenience in handling. In the latter event, the pigs, as desired, are reheated to a temperature of between 1250 degrees and 1350- degrees Fahrenheit for pouring into a die casting mechanism of conventional type.

Due to the short time for diffusion during solidification, sufficient deviation from equilibrium conditions are attained to permit a small amount of the hardening material, or eutectic to be retained at the grain boundaries of the solidified metal. Precipitation, of the eutectic, an aluminum magnesium compound, is later accentuated by a novel artificial aging process explained hereinafter. However, the amount of this material in the grain boundaries is not sufiicient to cause excessive hot-shortness or embrittlement of the die castings at elevated temperatures, since the aluminum content is preferably maintained at a value of from 1.0% to 1.5% by weight. It has been found that with an aluminum content in excess of 1.5%, hot-shortness or embrittlement at elevated temperatures becomes such that it would be very diificult to make certain die. castings of,

Aluminum 10-15% Manganese 0.5--1.0%v Nickel 0.005% max. Iron 0.005% max. Copper 0.05% max. Silicon 0.3% max. Total elements other than Mg, Al,

and Mn 0.3% max. Magnesium Balance The above alloy is then die cast and it has been found that by subjecting the die casting to a higher annealing temperature than customarily used for annealing or aging, and reducingthe time at such temperature, a maximum amount of precipitation of the intermetallic chemical compound of aluminum-magnesium could be obtained. This resulted in increased yield strength and hardness of castings of this alloy so treated, as compared to mechanical properties obtained by conventional heat treatment times and temperatures. The exact temperature and time used in this new heat treatment is four hours at 475 Fahrenheit. Good results may be obtained between the time range of three to five hours and the temperature range of'450 to 500 Fahrenheit. However, best results are attained by the use of the exact time and temperature given above. The latter process for heat treating the die casting to improve the grain structure thereof isdescribed and claimed in the copending application Serial No. 666,-'759,filed May 2, 1946, by Richard F. Hauser and assigned to Bendix Aviation Corporation. r v

Through this novelheat treating or artificial aging process the grain structure of the as cast condition illustrated in the drawing of Figure 1 isv converted into an approximate homogeneous equi-axed structure as shown for example in the drawing of Figure 2, which materially improves the mechanical properties of the material. The latter die cast magnesium alloy is described and claimed in the copending application Serial No. 666,759, filed May 2, 1946, by Richard F. Hauser, and assigned to Bendix Aviation Corporation.

The comparative mechanical properties of the same are as follows:

Yield strength is defined as the stress at which the stress-strain curve deviates .2% from the modulus line.

From the foregoing it will be readily seen that there has been provided a novel improved magnesium alloy and a novel heat treating or aging process for producing the same.

"While only two examples of the invention are illustrated herein, it is to be clearly understood that the invention is not to be limited thereto, but the same is to be considered merely as illustrative of the practice thereof, since the invention is notto be limited except by the appended claims.

What is claimed is:

1. In a method for producing a magnesium die casting having a substantially homogeneous equi-axed grain structure; the steps comprising heating a magnesium-manganese alloy to a temperature between about 1250 F. and about 1350 F.; dissolving in the molten alloy not in excess of 1.5% by weight of aluminum and an amount of aluminum sufficient to decrease the solubility of the manganese in the magnesium; precipitating the liberated manganese so as to cleanse the molten metal of suspended particles by causing such particles to sink to the bottom of the molten metal by an agglomeration with the liberated manganese, die casting the cleansed alloy, and subjecting the die casting to a temperature of between about 450 F. and about 500 F. for a period of between three to five hours so as to artificially age the casting and convert the structure of the die casting into an approximate homogeneous equi-axed grain structure, the aluminum dissolved in said molten alloy being in an amount not less than 1% by weight.

2. In a method for producing a magnesium die casting having a substantially homogeneous equiaxed grain structure; the steps comprising first,

heating a magnesium alloy having approximately 1.5% by weight of manganese to a temperature of not less than 1250 F. and not more than 1350 F.; second, dissolving in the alloy at said temperature between 1.00% and 1.5% by weight of aluminum so as to decrease the solubility of the manganese in the magnesium and effect through action of the liberated manganese a cleansing action upon said alloy; third, die casting the cleansed alloy; and fourth, subjecting the die casting to a temperature of between 450 to 500 F. for a period of between three to five hours. so as to artifically age the casting and convert. the structure of .the casting into 'an approximate homogeneous equi-axed grain structure.

. 3. In a method for producing a magnesium die casting having a substantially homogeneous equi-axed grain structureythe steps comprising first, heating a magnesium alloy having approximately 1.5% by weight of manganese to a temperature of between 1250 F. and 1350 F.; second, dissolving in the alloy at said temperature between 1.00% and 1.5% by weight of aluminum so as to decrease the solubility of the manganese in the magnesium and effect through action of the liberated manganese a cleansing action upon said alloy; third, die casting the cleansed alloy; and fourth, subjecting the die casting to a temperature of approximately 475 F. for approximately four hours so as to artificially age the casting and convert the structure of the casting into an approximate homogeneous equi-axed grain structure.

4. In a method for producing a magnesium die casting; the steps comprising first, heating a magnesium alloy having approximately 1.5% by weight of manganese to a temperature of between 1250 F. and 1350 F.; second, dissolving in the alloy at said temperature between 1.0% and 1.5% by weight of aluminum so as to decrease the solubility of the manganese in the magnesium; third, maintaining the alloy at said temperature for an appreciable period of time sufficient to permit suspended particles of impurities in said alloy to sink to the bottom of the alloy by an agglomeration with the liberated manganese so as to cleanse said alloy of said impurities; fourth, die casting said cleansed alloy; and fifth, subjecting said die casting to a temperature of between 450 F. and 500 F. for a period of between three to five hours.

5. In a method for producing a magnesium die cast alloy; the steps comprising first, heating a magnesium alloy having approximately 1.5% by Weight of manganese to a temperature of between 1250 F. and 1350 F.; second, dissolving in the alloy at said temperature between 1.0% and 1.5% by weight of aluminum so as to decrease the solubility of the manganese in the magnesium; third, maintaining the alloy at said temperature for an appreciable period of time sufiicient to permit suspended particles of impurities in said alloy to sink to the bottom of the alloy by an agglomeration with the liberated manganese so as to cleanse said alloy of said impurities and to retain in the alloy between .5% and 1.0% by weight of manganese, between 1.0% and 1.5% by weight of aluminum, and total elements other than magnesium, manganese and aluminum not in excess of .3% by weight; fourth, die casting said cleansed alloy; and fifth, subjecting said die cast alloy to a temperature of between 450 F. and 500 F. for a period of between three to five hours to convert the grain structure of the die cast condition of the alloy into a substantially homogeneous equi-axed grain structure.

RICHARD F. I-IAUSER.

(References on following page) 8 REFERENCES- CITED. FOREIGN PATENTS: The following references are of record in the Number Country Date. file of this patent: 524,113 Great'Britain July 30, 1940 UNITED STATES PATENTS Number OTHER REFERENCES Name 7 Date. Magnesium, 1946, American Soc. for Metals, Schmidt July 13, 1926 Cleveland, Ohio, pages 67 and 68. Gann May 22, 1934; Metals Handbook, 1939 edition, Am. $00. for Schmidt et a1. Feb- 4,1935 Metals, Cleveland, Ohio, page 1583. Hansell June 15, 19 40v 10 Technology of Magnesium and Its Alloys, Hanawalt. Dec 30, V 1941 1940, E. A. Hughes:& Co., London, England, pages Toppingl Mar; 16,1943 318-319.

Weber et al.- May 8, 1945. 

1. IN A METHOD FOR PRODUCING A MAGNESIUM DIE CASTING HAVING A SUBSTANTIALLY HOMOGENEOUS EQUI-AXED GRAIN STRUCTURE; THE STEPS COMPRISING HEATING A MAGNESIUM-MANGANESE ALLOY TO A TEMPERATURE BETWEEN ABOUT 1250* F. AND ABOUT 1350* F.; DISSOLVING IN THE MOLTEN ALLOY NOT IN EXCESS OF 1.5% BY WEIGHT OF ALUMINUM AND AN AMOUNT OF ALUMINUM SUFFICIENT TO DECREASE THE SOLUBILITY OF THE MANGANESE IN THE MAGNESIUM; PRECIPITATING THE LIBERATED MANGANESE SO AS TO CLEANSE THE MOLTEN METAL OF SUSPENDED PARTICLES BY CAUSING SUCH PARTICLES TO SINK TO THE BOTTOM OF THE MOLTEN METAL BY AN AGGLOMERATION WITH THE LIBERATED MANGANESE, DIE CASTING THE CLEANSED ALLOY, AND SUBJECTING THE DIE CASTING TO A TEMPERATURE OF BETWEEN ABOUT 450* F. AND ABOUT 500* F. FOR A PERIOD OF BETWEEN THREE TO FIVE HOURS SO AS TO ARTIFICIALLY AGE THE CASTING AND CONVERT THE STRUCTURE OF THE DIE CASTING INTO AN APPROXIMATE HOMOGENEOUS EQUI-AXED GRAIN STRUCTURE, THE ALUMINUM DISSOLVED IN SAID MOLTEN ALLOY BEING IN AN AMOUNT NOT LESS THAN 1% BY WEIGHT. 